Heparin causes thrombocytopenia in 1 - 3 per cent of patients and leads to thrombosis in about half of these. There have been several laboratory tests to diagnose this problem, but none of them is perfect and some are very labor intensive. Recently a new assay method has been published that offers the promise of high sensitivity and specificity but with relatively little labor. In order to collect a sufficient number of patient samples to study this new assay, we have collaborated with physicians at Washington Hospital Center, where heparin-induced thrombocytopenia (HIT) is much more common than here. Forty patients are being accrued with a spectrum of clinical probability of actually having HIT. Other patients are being accrued at the Clinical Center. Samples from these patients will be tested with the new laboratory test for HIT as well as the time-honored but difficult assay that we currently consider the best. If the new test performs well, we will discontinue our older method.